Leak-resistant dry cells

ABSTRACT

A leak-resistant dry cell comprises a negative electrode formed by a zinc tube which is designed to be consumed when the cell voltage has fallen to e.g. 0.75 volt, a sealed bag of flexible plastics material enclosing said zinc tube and surrounded by a cylindrical jacket, a carbon positive electrode projecting through said bag and sealed thereto, and a tongue extending from the edge of the sidewall of the zinc tube through said bag in sealing relationship therewith, the tongue contacting a negative terminal of the cell.

The present invention is concerned with leak-resistant dry Leclanchecells, and more particularly to dry cells of the cylindrical type havinga reduced zinc content compared to conventional dry cells.

It is well known that leakage of dry cells near or after the end oftheir useful life is objectionable, and various proposals have been putforward to control such leakage. These proposals, although sometimesquite effective, have been costly to implement.

Leakage becomes a major problem when attempts are made to reduce thezinc content of a dry cell, since it is then to be expected that thezinc negative electrode (properly termed the anode where a primary cellis concerned) will be extensively perforated by the time that the cellis exhausted. On the other hand a reduction in the zinc content doeshave the advantage that when the zinc is used up electrochemicalreactions in the cell virtually cease, and hence the production ofcorrosive liquid within the exhausted cell is much reduced. It will beappreciated that any excess of zinc left in a cell when its voltage hasdropped below a minimum acceptable value, e.g. 0.75 volt, has no actualuse. If the circuit is still switched on or there is a short circuit,this excess of zinc can still undergo electrochemical reaction insidethe cell until all the zinc is consumed, the result being the exuding ofa large quantity of liquid. If the amount of zinc is controlled so thatit will be substantially used up when the voltage of the cell reachese.g. 0.75 volt, the electrochemical reaction drops to zero rapidly andthus no more liquid will be generated.

When we control the amount of zinc used, we have to make the zincelectrode very thin since the amount of zinc used is small. When theterminal voltage has dropped to around 1 volt, there is not much zincleft in the zinc electrode. In fact at this time the jacket of the cellis in contact with the electrolyte. Therefore the jacket must be madewith material which can withstand the corrosive liquid of theelectrolyte. Self-corrosion of the zinc electrode occurs in a lot ofcells before their usage and leakage also occurs before actual usage. Ifa cell is to utilise a thin zinc electrode, than a corrosion-proofjacket is essential.

If the zinc electrode is made very thin, then there will be a problem ofkeeping the remainder of the zinc in position and in good contact withthe electrolyte when it is almost used up. Before the zinc electrode ismostly used up, there will be some liquid exuded and therefore thejacket must have enough internal space to contain this liquid. Theinternal pressure will also increase and the outer jacket must standthis high internal pressure for a long time.

Our invention is based on the realisation that a leak-resistant dry cellof reduced zinc content can advantageously incorporate a soft plasticsbag to control leakage and thus avoid damaging appliances of high valuein which the battery may be placed.

One aspect of our invention provides a dry cell comprising a negativeelectrode formed by a zinc tube which is open at both ends and is sodimensioned that it is substantially totally consumed when the cellvoltage has fallen to a predetermined minimum value e.g. in the range0.70 to 0.85 volt, a sealed bag of flexible plastics material enclosingsaid zinc tube and surrounded by a cylindrical jacket, a carbon positiveelectrode (properly termed a cathode in connection with a primary cell)projecting through said bag in sealing relation thereto, and at leastone tongue extending from said zinc tube through said bag in sealingrelation thereto into contact with a negative terminal of the cell.

In order to conserve zinc, the height of the zinc tube will generally besubstantially the same as that of the depolarising dolly within thecell. The jacket may be of any suitable material, e.g. metal, paper, orcardboard; the base of the cell will ordinarily comprise a metalnegative terminal which may be attached to the cylindrical jacket. Thecarbon electrode and the tongue extending from the zinc tube must form aseal with the bag effective to prevent passage of liquid during storageand use of the cell. Suitable sealing materials, as mentionedhereinafter, may be used to ensure this.

According to a further aspect of our invention the above-defined drycell may be made by a process comprising the steps of sealing said zinctube containing a dolly and electrolyte in a bag of flexible plasticsmaterial, surrounding said bag by said cylindrical jacket and insertinga carbon electrode into said dolly through said bag. The advantages ofconstructing the cell in this way will be described hereinafter.

Although it is possible to insert the dolly and electrolyte into apreformed zinc tube, we prefer to form the tube from zinc sheet aroundthe dolly and electrolyte; this is operationally convenient and resultsin excellent contact between the zinc electrode and the electrolyte.This method is particularly preferable in the production of paperlinedcells. If the zinc tube is formed by rolling up a zinc sheet, thelongitudinal or helical line of abutment need not be closed by solderingor welding, since the plastics bag will prevent leakage or drying up ofthe electrolyte. The tongue projecting from the zinc tube may beintegral or may be attached thereto in electrically conductive manner.

The bag of flexible plastics material is preferably of polyolefinmaterial such as polyethylene or polypropylene or of vinyl material suchas PVC. It may incorporate an aperture to admit the tongue projectingfrom the zinc tube or the tongue may pierce an aperture when the tube,dolly and electrolyte assembly is placed in the plastics bag. The bagmay be preformed and the tube, dolly and electrolyte assembly placedtherein or the bag may be formed around said assembly, e.g. by sealingboth ends of a length of plastics tubing.

Two embodiments of our invention will now be described by way of exampleonly with reference to the accompanying drawings, wherein:

FIG. 1 shows an elevational view of a flexible plastics bag suitable foruse in this invention with its opening at the bottom (as drawn).

FIG. 2 shows an elevational view of another type of flexible plasticsbag with a lateral opening facing sideways (as drawn).

FIG. 2a shows a modification of the bag of FIG. 2 in which the hole forthe passage of the tongue extending from the electrode is made bycutting off a corner of the bag.

FIG. 3 shows a cross-sectional elevational view of the plastics bag ofFIG. 1 enclosing the depolarizing mix, electrolyte in the form ofelectrolytic paste lined paper and the zinc electrode. The bag isalready sealed.

FIG. 4 corresponds to FIG. 3 except that the bag of FIG. 2 has beenemployed.

FIG. 5 shows a cross-sectional elevational view of a completed cellsurrounded by a paper jacket.

FIG. 6 shows a cross-sectional elevational view of a completed cellsurrounded by a metal outer jacket.

In the embodiments illustrated the zinc electrode contains a controlledamount of zinc which will be substantially used up when the voltage ofthe cell has dropped to below about 0.75 volts. Then the only liquidexuded inside the cell is formed before this stage, and very little isformed after this stage. The amount of liquid exuded is limited andtherefore leakproofing can easily be done.

In all the figures 1 is the zinc tongue which extends from the thin zincelectrode for the purpose of conductivity. The zinc electrode inconventional cells is large in area and in volume and is hard to wrapand seal, but with a narrow zinc tongue, it can be wrapped and sealedeasily and does not affect the conductivity.

Referring now to FIG. 1, there is shown a flexible plastics bag with anopening facing downward, the top edge 4 and sides 5 being sealed.Flexible plastics materials can withstand the corrosive liquid exudedinside a cell during use. Many attempts have been made to use flexibleplastics film to wrap and seal a dry cell, but because of its strongelastic nature, it is hard to work with during manufacture.

In another embodiment the flexible plastics bag is open along one sideas in FIG. 2, the other three sides being sealed except for a small hole6 in the lower corner of the bag where the zinc tongue will extendthrough. Sealing materials will be applied at 7 for the sealing of thetongue-way 7.

As shown in FIG. 2a, the hole 6 may alternatively be made by cutting offa corner of the bag. Sealing materials will again be applied at 7.

In constructing the cell of FIG. 5 a piece of paper 8 which is saturatedwith electrolytic paste is applied to a flat piece of zinc sheet; thiszinc sheet will form the negative electrode 2. The electrolytic pastecan be solidified before or after the paper is applied to the zinc.

Then the zinc with the paper is rolled up to surround the depolarizingdolly 9 and so yield the active combination of the battery. This activecombination is then placed inside the flexible plastics bag of FIGS. 1,2 or 2a and the opening of the bag is sealed by heat or high frequencywelding along the dotted line 10 in FIGS. 1 to 4. In actual production,we prefer to seal the hole 6 individually which the zinc tongue extendsthrough by applying adhesive materials such as asphalt, wax, sealingwax, silicone rubber or resin at 6 and 7 to seal up the hole 6. We canalso apply the above materials on both sides of the zinc tongue 1 beforethe active combination is placed in the soft plastics bag. After the bagis sealed by heat or high frequency welding at line 10, we only have topress the plastics bag at 6 and 7, to complete the sealing of the hole6.

The above way of sealing has one special advantage; when the zincelectrode 2 is used up, then the corrosive liquid inside the batterywill be in contact with the zinc tongue 1. If the zinc tongue 1 is notprotected by the above mentioned sealing materials, the liquid willcorrode away the zinc tongue 1 rapidly until liquid can escape throughthe hole 6. But when the zinc tongue is surrounded and protected by thesealing material, the corrosive liquid cannot attack the whole zinctongue 1 and the leakage through the hole 6 is substantially eliminated.The same effect can be achieved by pre-treating the tongue 1 with avarnish or lacquer, in which case the hole 6 may be heat sealed.

Alternatively the active combination may be enclosed in a plastics bagwith a lateral opening 3 as in FIG. 2. The sealing along line 10, as ofthe aperture 6 and 7 in FIG. 4 is performed as described above inrelation to FIG. 3. The active combination is placed in through thelateral opening in the plastics bag and the zinc tongue is extendedthrough 6 as shown in FIG. 4.

The active combination in FIG. 3 or 4 is specially designed, and it willbe seen that when the bag is sealed, the active combination is stillwithout a carbon positive electrode.

The sealed plastics bag of FIG. 3 or 4 is then properly positioned in anouter jacket 11 (FIGS. 5 and 6). The outer jacket can be made of paperor metal or other material without consideration of corrosion caused bythe liquid exuded because all the liquid will be retained inside theplastics bag. The carbon rod 12 is pressed down from the top all the wayto the bottom of the depolarizing dolly 9 along its longitudional axis.

When the carbon rod 12 is driven into the depolarizing dolly 9, thevolume of the dolly is bound to expand. But its circumferentialexpansion is limited by the bottom plate 13 which is supported by thecell assembly machine. Because the expansion of the dolly is limited inall directions the pressure exerted on the dolly by driving in thecarbon rod 12 will only increase its hardness. This pressure will alsobe transmitted to the electrolytic paste lined paper 8 and the zincelectrode 2 so that they will be pressed against the paper or metalouter jacket and formed into the same circular shape. The driving of thecarbon rod into the depolarizing dolly will also exert even pressure onall the elements from the dolly to the electrolytic paste lined paper tothe zinc electrode, so that the internal resistance is reduced.

The above mentioned sealing materials are then applied around the spot14 where the carbon rod 12 enters the depolarizing dolly 9. The zincnegative electrode 2 is backed up and supported by the outer jacket 11,so the fact that the zinc will be perforated during use will be of noconsequence. Sealing of the plastics bag which contains the activecombination before the carbon rod is driven into the depolarizing dollyis a noteworthy novel aspect of our invention.

When the carbon rod 12 is driven through the plastics bag at 14 into thedepolarizing dolly, the adjacent area of the plastics bag will exert acertain amount of pressure on the depolarizing dolly and this willdecrease the internal resistance in novel manner.

Another feature of this invention is that when the plastics bag issealed, in our design it will transform from its flat shape into acylindrical shape, with the result that four irregular pyramidal pockets15 are formed two on each of the two ends of the soft plastics bag.These pockets can expand to accomodate the liquid exuded inside thebattery. In order to make use of these pockets 15, we have to expel asmuch air as possible from these pockets before the plastics bag issealed.

In conventional cells, the construction itself has to stand up to theever-increasing internal pressure besides doing its function ofleakproofing. The plastics bag of this invention has only to prevent theliquid from leaking out. The outer jacket 11 will resist the internalpressure. Also there are four pyramidal pockets 15 which can store theliquid exuded inside the cell so as to reduce the internal pressure;therefore, the bag itself will not burst open due to the increasing highinternal pressure. The outer jacket 11 must be strong enough to standthe higher but limited inner pressure of the cell.

The sealing of the bag in this invention is a clean and simpleoperation; the sealing area is rarely contaminated by raw materialsduring manufacturing. Also, because the carbon rod 12 is driven into theenclosed and sealed depolarizing dolly 9, the top end of the carbon rod12 is always very clean and corrosion of the metal top cover 16 due toelectrolyte contamination is substantially eliminated.

After putting on the metal top cover 16 and the metal bottom plate 17,the upper and lower edges 18 of the paper outer jacket 11 are rolledinward as in FIG. 5 and a paper jacketted leak-resistant cell iscompleted.

If a metal jacket is preferred, two insulating cardboard or plasticsrings 19 are required after the metal top cover 16 and the metal bottomplate 17 are put in place as in FIG. 6. Then the upper and lower edge 18of the metal jacket 11 are rolled or pressed in to complete the metaljacketted leak-resistant cell of FIG. 6.

We claim:
 1. A method of making a cell comprising a negative electrodein the form of a zinc tube having a tongue extending from a sidewall ofthe tube, the tongue being in contact with a negative terminal of thecell, a dolly and electrolyte within the tube, a sealed, flexible bag ofplastic material enclosing the tube with the tongue extending externallyof the bag and in sealing relationship therewith, a cylindrical jacketsurrounding the tube externally of the bag, and a positive carbonelectrode extending through the bag in sealing relationship therewithand into the dolly; comprising the steps of:(a) placing the zinc tube,the dolly and the electrolyte within a soft, flexible bag of plasticsmaterial; (b) sealing the plastics bag closed to totally enclose thetube, dolly and electrolyte, with the tongue extending externally of thebag, and including sealing the tongue relative to the bag; (c) placingthe cylindrical jacket about the tube externally of the bag; (d)inserting the carbon electrode into the dolly through the sealed bag;and (e) sealing the carbon electrode relative to the bag.
 2. The methodof claim 1 wherein the bag is provided with a relatively large openingand a relatively small opening, and wherein step (a) comprises insertingthe zinc tube, dolly and electrolyte into the bag through the largeropening and causing the tongue to extend outwardly of the bag throughthe smaller opening; and wherein the step (b) is carried out by sealingthe larger opening of the bag closed and sealing the tongue relative tothe bag at the smaller opening area.
 3. The method of claim 1 whereinthe step (b) of sealing the tongue relative to the bag involves the useof a pressure sensitive adhesive.
 4. The method of claim 1 wherein saidzinc tube is formed by rolling zinc sheet around said dolly.
 5. Themethod of claim 1 wherein said bag is formed of a material selected fromthe group consisting of polyolefin materials and vinyl materials.
 6. Themethod of claim 1 wherein said tongue and said carbon anode are sealedto said plastics bag by an adhesive material selected from the groupconsisting of asphalt, wax, sealing wax, silicone rubber and resin. 7.The method of claim 6 wherein said adhesive is applied to both sides ofthe tongue before the zinc tube is sealed to the plastics bag.